My research projets are divided in two different lines of research which are briefly explained below.

Glass transition and properties of amorphous solids

In general, when the temperature of a liquid is reduced, the material undergoes a phase transition and cristalizes. A possible approach to understand this transition is to use statistical mechanics and show that, below a certain temperature, the free energy of the system is smaller in the crystal phase than it would be if the material were in the liquid phase. In this kind of description, the time scales at which the temperature is reduced are not taken into account ; it is simply assumed that the material is at thermal equilibrium along the process.

However, this condition is not always respected. Actually, many systems in our daily life are out of equilibrium. Well known examples are biological systems, which are open systems subjet to fluxes of energy. Another important example are glasses. The dynamical processes with which these solids are generated became very important when the temperature is decreased so fast that the molecules do not have time to organize and the resulting material has an amorphous character. These materials then relax so slowly that they cannot equilibrate in a time scale observable in the laboratory. This material is called "amorphous solid" and the process that generates it is called "glass transition". The mechanisms of this transition are not completly understood and the theoretical descrition of it is one of the biggest chalenges in the field of soft matter.

Beyond the intriguing phenomenology behind the mechanisms responsible for the glass transition, there are a set of challenging questions about the properties of the amorphous material. These materials present different properties in respect to crystal, as for example the force chain and its vibrational modes (Figure 2). These two facts have important implications in the nature of transport properties, linear response of these systems and the dynamics during the glassy phase.

Fig.1: Vibrational modes of hard spheres in 2d. In this figure there are few modes of very low frequency. Left: In the crystal phase, the modes are plane waves. Some modes are degenerescent due to the symmetry of the system. Right: in the amorphous phase, the vibrational modes are very different from plane waves. Besides being spatially heterogeneous, its frequency are smaller (the value of the frequencies are written in the legend inside the modes).




Nanoporous alumina arrays (NAA)

The nanoporous alumina arrays (NAA) formed by aluminum anodization can be considered to be the most popular self-organized hexagonal packing system. The high interest in this methodology is mainly due to its simplicity and low cost. For many engineering applications, such as high-density magnetic recording media, photonic crystals, or pattern-transfer masks, the ordering and organization of the NAA is a crucial factor. When a high degree of regularity and uniformity is required for applications, it is very important to quantify the level of ordering in a NAA structure. In a recent work, we used an approach inspired by the theory of two-dimensional melting, developed to describe phase transitions in two-dimensional systems that present liquid-crystal-like structures. We defined a local order parameter (LOP) to quantify the degree of hexagonal order of each pore without any arbitrary parameters.

Fig.2: Samples of NAA produced in the F3Lab- IF/UFRGS. They were produced using different parameters (acid type and concentration, voltage) and as a result the have different spatial order.


How to quantify order in Nanoporous Alumina Array (NAA)?


We used statistcal tools to develop a method to quantify the degree of hexagonal order in samples of NNA. The approach is inspired by the theory of two-dimensional melting, developed to describe phase transitions in two-dimensional systems that present liquid-crystal-like structures. The approach is explained in detail in this paper and can be trivially extended to characterize other physical systems that form hexagonal packings. We also developed a web site where everyone can use this method to quantify order in its own sample.


Superhydrofobicity

Another application of these surfaces is the property of superhidrofobicity, which means the contact angles of a water droplet exceeds 150° and the roll-off angle/contact angle hysteresis is less than 10°. In general lines, to be superhidrofobic, a surface needs to have two characteristics: (i) low surface tension and (ii) rugosity in many scales. Since the nanoporous introduce rugosity in nanoscale, it can enhance the hydrofobicity of the surface. An open question is if the spatial order of the pores have an influence in this property.